The Wild Kratts dive into the ocean to visit Poseidon's temple. They are well received by him (after noticing they can breathe normally in the temple, despite all the water around), but suddenly, Martin's medallion starts dragging him to somewhere. Three swordfish (Poseidon's fish-keteers) plus a younger one slow him down, helping him to stop.

Martin follow his medallion's direction, and opens a secret compartment with a sword on it. Poseidon tells him that the sword on e belonged to the previous Water Guardian, and shocks Martin by telling that according to the Musketeer Law, he must stay in the temple forever.

A seahorse comes with a message from Mother Nature, telling them that there are suspects of a well-known villain around the place, searching for swordfishes. The crew recognize Gourmand's description. Meanwhile, on the surface, Gourmand prepared himself to dive, wanting to get swordfish for a soup.

Later in the temple's garden, trying to cheer up Martin, the younger of the swordfish, asks to train with him, and tell him about his past, when he was saved by the Fish-Keteers. Martin remembers of The Three Musketeers story after hearing that, and the swordfish believes that Poseidon already read the book, due to his brothers' names, Athos, Porthos and Aramis. Martin decides to name him D'Artagnan, a name that the swordfish approves.

While the fish-keteers discuss a way to protect the temple, Gourmand sneaks into the temple, not knowing that a dark magic cloud just entered with him, hiding in his hat.

When night comes, the Wild Kratts sleep on their room, except for Martin, still bugged about the idea of leaving the ones he love. He goes for a walk around the temple, and D'Artagnan, bugged by his elder brothers not giving him a chance of joining them during patrol, finds him. They both talk about their problems and decides speaking with Poseidon by the morning. Suddenly, D'Artagnan hears one of his brothers calling for help.

When the young swordfish and the Wild Kratts reach the place, they find Aramis, who tells that someone kidnapped Athos and Porthos, and that he escaped by hiding. A seahorse swims on their direction, telling that Poseidon's trident was stolen by what seemed to be a black cloud. Recognizing the descriptions, the Wild Kratts run to their vehicles to find Gourmand and the dark cloud. Martin, despite the law, affirms that he's still the Water Guardian, so he must go with them.